Due to resource attrition and pollution, there exists a need to supplant conventional carbon-based energy sources such as coal, oil, and natural gas with renewable sources such as solar energy. A limiting factor in the utilization of solar energy is the high cost of energy converters such as solar cells (e.g., photoelectric elements such as photovoltaic (“PV”) cells). Another limiting factor in the utilization of solar energy is the uneven availability of incident solar radiation (illumination) from one season to another, one day to another, and even within a day (e.g., due to variation in weather). Illumination also varies dramatically with respect to latitude. It would be desirable to maximize output and reliability of PV cell-based systems over a wide range of illumination conditions.
Two basic approaches for enhancing output of PV systems include (A) improving efficiency of solar cells; and (B) concentrating incident radiation (e.g., using lenses, mirrors, parabolic dishes, or other optics—often including a mechanism to track the path of the sun during the day. PV efficiency is limited (e.g., silicon-based PV cells may have maximum theoretical efficiency of ˜29%), PV cell efficiency declines with increasing temperature, and highly efficient PV cells are expensive. Solar concentrators deliver increased radiation to solar cells, enabling greater power output from a given number of cells, typically at a lower cost than simply deploying more cells not subject to concentration. Depending on the concentrating elements used, incident radiation may be concentrated only slightly, or greatly˜e.g., to a factor of 1000 or more (e.g., as described in U.S. Pat. No. 6,717,045, which is hereby incorporated by reference).
One advantage of highly concentrated PV systems is that they enable generation of substantial power early and late in the day, when illumination intensity is low. Such systems, however, may transmit too much radiation to PV cells at mid-day when illumination is at its maximum, and require specialized cooling systems to transfer excess heat away from the solar cell. Such cooling systems may be expensive to construct or operate, may require frequent maintenance, may degrade in efficiency, and/or may have slow transient response. Moreover, failure of a cooling system in a highly concentrated system may reduce useful life of a PV cell life. It would be desirable to overcome these limitations.
A solar concentrator typically utilizes a fixed concentrating element, and exhibits a fixed concentration factor. Providing concentrators with optimal concentration factors for different locations and/or times of year may be difficult, as higher concentration factors may be appropriate for locales and periods receiving low average or peak illumination intensity (e.g., latitudes closer to the poles, winter season in the northern hemisphere, etc.) versus locales and periods characterized by greater average or peak illumination. While moveable optical components (i.e., moveable relative to a PV cell) could be employed to alter concentration factor, addition of moving parts would increase cost, increase maintenance requirements, and may compromise reliability by introducing additional failure modes. It would be desirable to obtain the benefits of highly concentrated PV systems during low to moderate illumination conditions, and avoid drawbacks of excess concentration during high illumination conditions, without requiring relative movement between optical components and a PV cell. It would be further desirable to standardize a solar concentrator without requiring different concentration factors for different locations and periods.
Up to a limit, an increase in illumination on a PV cell causes an increase in converted power. There is a point, however, at which efficiency of the cell becomes so poor due to rise in temperature that any increase in illumination causes a decrease in converted power. The point at which this occurs (“maximum power conversion point”) represents the maximum power that a cell can convert. (See Yates, Tarn A., thesis entitled “Solar cells in concentrating systems and their high temperature limitations,” September 2003, available at http://quantum.soe.ucsc.edu/research/SolarCell/Tarn_Senior%20Thesis.pdf, hereby incorporated by reference.) It would be desirable to maintain a PV cell at (or not exceeding) its maximum power conversion point.
In some cases, solar concentrators fail to provide optimally even distribution of concentrated solar radiation on a PV cell. That is, radiation may be more concentrated along one portion (e.g., center) of a solar cell than along another portion (e.g., edges), potentially resulting in decreased electrical output and uneven thermal loading across the surface of the cell. Additionally, useful life of a solar cell in a concentrated system may be reduced by extended operation at very high temperatures. It would be desirable to overcome these limitations.
Based on the foregoing, the art continues to seek improvements concentrated solar energy utilization systems.